Infrared fiber optic waveguides are needed for transmission in the 2 to 10 micron (micrometer) wavelength region. These are required for focal plane arrays and remote sensing. They are also required for flexible delivery of high power carbon monoxide and carbon dioxide laser energy as well as for extremely low-loss long distance communication links. A predominate consideration in the formation of optical fibers is the surface, which must be smooth for the efficient transmission of optical signals. Scattering losses during multiple internal reflections cannot be tolerated so a smooth surface must be achieved during fabrication.
Extrusion of some materials suitable for use as infrared optical fibers has been successful. For example, thallium bromoiodide has been successfully extruded with a sufficiently smooth surface. Starting with thallium bromoiodide crystals of 5.3 millimeters diameter, long lengths of fiber have been extruded to diameters of 75 to 500 microns. This example shows extrusion reductions of 5,000:1 and 112:1 respectively. In that process, the crystal billets are compressed in a heated die of tungsten carbide and extruded through a diamond orifice. The fiber is wound on a large spool. Other materials which are suitable for infrared transmission and are extrudable include silver chloride, silver bromide and potassium bromoiodide. Success in extruding thallium bromoiodide crystals led to attempts to extrude potassium chloride and potassium bromide, which are suitable optical fiber materials for infrared transmission and are capable of even lower loss in transmission. However, the surface of such materials, as extruded, resembled fish scale or shingle-like surface which causes internal scattering losses which are unacceptable in optical fiber utilization. Increased extrusion temperatures and changes in extrusion speed were ineffective in improving the potassium chloride fiber surface. An example of an extruded silver bromide fiber clad with silver chloride is found in Anderson et al U.S. Pat. No. 4,253,731. In addition, Anderson et al, U.S. Pat. No. 4,271,104, teaches hot rolling or extrusion of a sheet of material which can be used as a ribbon of a plurality of adjacent fibers. However, Anderson was not able to achieve a sufficiently smooth rolled surface, even on extrudable materials, for optical fibers. This patent also speaks of silver chloride and silver bromide as suitable fiber material.
The fact that thallium bromoiodide and silver chloride are extrudable and the higher melting point potassium chloride and potassium bromide are not, is derived from the extrusion process as related to the material properties. The large area reductions of 5,000 to 1 require that the core of the extruding billet travel at a rate 5,000 times faster than the surface as the fiber is formed. This difference in movement, coupled with high frictional forces at the surface of the billet and in the forming orifice, causes a fragmentation of the crystal structure which cannot be compensated by available crystal slip systems. The material comes out of the extrusion die with a fish-scale surface. Low reduction ratios are needed but are not practical in extrusion because at small diameters the extrusion piston must be of long length and small diameter, in dimensions not able to withstand the stresses of extrusion. Only the very ductile thallium bromoiodide and similar structures can reconsolidate into cohesive fiber during large deformations. On the other hand, potassium chloride and potassium bromide surfaces tear in tension and flatten into fish-scale form. Thus, there is a need for a method and apparatus for forming potassium chloride, potassium bromide and other such materials into optical fibers.